Heat-sealable films

ABSTRACT

A multilayer polyolefin film of the type suitable for packaging application in which heat seals are formed, and in its preparation the multilayer film comprises a flexible substrate layer formed of a crystalline thermoplastic polymer having an interface surface. A heat-sealable surface layer is bonded to the interface surface of the substrate layer and is formed of a syndiotactic propylene polymer effective to produce a heat seal with itself at a sealing temperature of less than 110° C. The surface layer has a thickness which is less than the thickness of the substrate layer. The heat-seal layer can be formed of syndiotactic polypropylene polymerized in the presence of a syndiospecific metallocene catalyst and having a melt flow index of less than 2 grams/10 minutes. The multilayer film can take the form of a biaxially-oriented film. In the production of the multilayer film incorporating a substrate layer and a heat-sealable surface layer, a crystalline thermoplastic polymer is extruded and formed into a substrate layer film. A second polymer comprising a syndiotactic propylene polymer which is effective to form a heat-sealable surface layer is extruded to form a surface layer that is bonded to the interface of the substrate layer at a temperature within the range of 150-260° C.

BACKGROUND OF THE INVENTION

Multilayer polyolefin films incorporate a base or a substrate layer of astereoregular crystalline thermoplastic polymer and one or more surfaceplies which can be formed on one or both sides of the base layer.Isotactic polypropylene is one of a number of crystalline polymers thatcan be characterized in terms of the stereoregularity of the polymerchain. Various stereospecific structural relationships denominatedprimarily in terms of syndiotacticity and isotacticity may be involvedin the formation of stereoregular polymers from various monomers.Stereospecific propagation may be applied in the polymerization ofethylenically unsaturated monomers such as C₃+ alpha olefins, 1-dienessuch as 1,3-butadiene, substituted vinyl compounds such as vinylaromatics, e.g. styrene or vinyl chloride, vinyl chloride, vinyl etherssuch as alkyl vinyl ethers, e.g, isobutyl vinyl ether, or even arylvinyl ethers. Stereospecific polymer propagation is probably of mostsignificance in the production of polypropylene of isotactic orsyndiotactic structure.

Isotactic polypropylene is conventionally used in the production ofrelatively thin films in which the polypropylene is heated and thenextruded through dies and subject to biaxial orientation by stressingthe film in both a longitudinal direction (referred to as the machinedirection) and in a transverse or lateral direction sometimes referredto as the “tenter” direction. The structure of isotactic polypropyleneis characterized in terms of the methyl group attached to the tertiarycarbon atoms of the successive propylene monomer units lying on the sameside of the main chain of the polymer. That is, the methyl groups arecharacterized as being all above or below the polymer chain. Isotacticpolypropylene can be illustrated by the following chemical formula:

Stereoregular polymers, such as isotactic and syndiotacticpolypropylene, can be characterized in terms of the Fisher projectionformula. Using the Fisher projection formula, the stereochemicalsequence of isotactic polypropylene as shown by Formula (2) is describedas follows:

Another way of describing the structure is through the use of NMR.Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . .with each “m” representing a “meso” dyad, or successive methyl groups onthe same side of the plane of the polymer chain. As is known in the art,any deviation or inversion in the structure of the chain lowers thedegree of isotacticity and crystallinity of the polymer.

In contrast to the isotactic structure, syndiotactic propylene polymersare those in which the methyl groups attached to the tertiary carbonatoms of successive monomeric units in the polymer chain lie onalternate sides of the plane of the polymer. Using the Fisher projectionformula, the structure of syndiotactic polypropylene can be shown asfollows:

Syndiotacticity can be characterized in terms of the syndiotactic pentadrrrr in which each “r” represents a racemic dyad. Syndiotactic polymersare semi-crystalline and, like the isotactic polymers, are essentiallyinsoluble in xylene. This crystallinity distinguishes both syndiotacticand isotactic polymers from an atactic polymer, which is non-crystallineand highly soluble in xylene. An atactic polymer exhibits no regularorder of repeating unit configurations in the polymer chain and formsessentially a waxy product.

For many applications the preferred polymer configuration will be apredominantly isotactic or syndiotactic polymer with very little atacticpolymer. Catalysts that produce isotactic polyolefins are disclosed inU.S. Pat. Nos. 4,794,096 and 4,975,403 to Ewen. These patents disclosechiral, stereorigid metallocene catalysts that polymerize olefins toform isotactic polymers and are especially useful in the polymerizationof highly isotactic polypropylene. As disclosed, for example, in theaforementioned U.S. Pat. No. 4,794,096, stereorigidity in a metalloceneligand is imparted by means of a structural bridge extending betweencyclopentadienyl groups. Specifically disclosed in this patent arestereoregular hafnium metallocenes that may be characterized by thefollowing formula:R″(C₅(R′)₄)₂HfQp  (4)In Formula (4), (C₅ (R′)₄) is a cyclopentadienyl or substitutedcyclopentadienyl group, R′ is independently hydrogen or a hydrocarbylradical having 1-20 carbon atoms, and R″ is a structural bridgeextending between the cyclopentadienyl rings. Q is a halogen or ahydrocarbon radical, such as an alkyl, aryl, alkenyl, alkylaryl, orarylalkyl, having 1-20 carbon atoms and p is 2.

Metallocene catalysts, such as those described above, can be used eitheras so-called “neutral metallocenes” in which case an alumoxane, such asmethylalumoxane, is used as a co-catalyst, or they can be employed asso-called “cationic metallocenes” which incorporate a stablenon-coordinating anion and normally do not require the use of analumoxane. For example, syndiospecific cationic metallocenes aredisclosed in U.S. Pat. No. 5,243,002 to Razavi. As disclosed there, themetallocene cation is characterized by the cationic metallocene ligandhaving sterically dissimilar ring structures that are joined to apositively-charged coordinating transition metal atom. The metallocenecation is associated with a stable non-coordinating counter-anion.Similar relationships can be established for isospecific metallocenes.

Catalysts employed in the polymerization of alpha-olefins may becharacterized as supported catalysts or unsupported catalysts, sometimesreferred to as homogeneous catalysts. Metallocene catalysts are oftenemployed as unsupported or homogeneous catalysts, although, as describedbelow, they also may be employed in supported catalyst components.Traditional supported catalysts are the so-called “conventional”Ziegler-Natta catalysts, such as titanium tetrachloride supported on anactive magnesium dichloride as disclosed, for example, in U.S. Pat. Nos.4,298,718 and 4,544,717, both to Mayr et al. A supported catalystcomponent, as disclosed in the Mayr '718 patent, includes titaniumtetrachloride supported on an “active” anhydrous magnesium dihalide,such as magnesium dichloride or magnesium dibromide. The supportedcatalyst component in Mayr '718 is employed in conjunction with aco-catalyst such and an alkylaluminum compound, for example,triethylaluminum (TEAL). The Mayr '717 patent discloses a similarcompound that may also incorporate an electron donor compound which maytake the form of various amines, phosphenes, esters, aldehydes, andalcohols.

While metallocene catalysts are generally proposed for use ashomogeneous catalysts, it is also known in the art to provide supportedmetallocene catalysts. As disclosed in U.S. Pat. Nos. 4,701,432 and4,808,561, both to Welborn, a metallocene catalyst component may beemployed in the form of a supported catalyst. As described in theWelborn '432 patent, the support may be any support such as talc, aninorganic oxide, or a resinous support material such as a polyolefin.Specific inorganic oxides include silica and alumina, used alone or incombination with other inorganic oxides such as magnesia, zirconia andthe like. Non-metallocene transition metal compounds, such as titaniumtetrachloride, are also incorporated into the supported catalystcomponent. The Welborn '561 patent discloses a heterogeneous catalystthat is formed by the reaction of a metallocene and an alumoxane incombination with the support material. A catalyst system embodying botha homogeneous metallocene component and a heterogeneous component, whichmay be a “conventional” supported Ziegler-Natta catalyst, e.g. asupported titanium tetrachloride, is disclosed in U.S. Pat. No.5,242,876 to Shamshoum et al. Various other catalyst systems involvingsupported metallocene catalysts are disclosed in U.S. Pat. No. 5,308,811to Suga et al and U.S. Pat. No. 5,444,134 to Matsumoto.

The polymers normally employed in the preparation of biaxially-orientedpolypropylene films are usually those prepared through the use ofconventional Ziegler-Natta catalysts of the type disclosed, for example,in the aforementioned patents to Mayr et al. Thus, U.S. Pat. No.5,573,723 to Peiffer et al discloses a process for producingbiaxially-oriented polypropylene film having a base layer formed of anisotactic polypropylene homopolymer or propylene ethylene co-polymers.Other co-polymers of propylene and alpha-olefins having from 4-8 carbonatoms also may be employed in the Peiffer process. Thus, the base layermay take the form of a mixture of isotactic polypropylene or ethylenepropylene copolymers with resin polymers such as styrene homopolymershaving a softening point of about 130-180° C. The surface layer orlayers may likewise take the form of a propylene homopolymer orcopolymer of the same type employed in the base layer.

Processes for the preparation of biaxially-oriented polypropylene filmsemploying polymers produced by the use of isospecific metallocenesinvolving di- or tri-substituted indenyl groups are disclosed inCanadian Patent Application No. 2,178,104. Four isotactic polymersdisclosed there are based upon the polymerization of propylene in thepresence of heavily substituted bis(indenyl) ligand structures. In eachcase, the metallocene used was a silicon-bridged di-or tri-substitutedbis(indenyl) zirconium dichloride. More specifically, the metallocenecatalysts used are identified in the aforementioned Canadian patent asrac-dimethylsilanediethyl bis(2-methyl-4,6 diisopropyl-1 indenyl)zirconium dichloride, 2 rac-dimethylsilanediethylbis(2-methyl-4,5-benzo-1-indenyl) zirconium dichloride, 3rac-dimethylsilanediethyl bis(2-methyl-4-phenyl-1-indenyl) zirconiumdichloride, and 4 rac-dimethylsilanediethylbis(2-ethyl-4-phenyl-1-indenyl) zirconium dichloride. The variouspolymers produced by these metallocenes catalysts are characterized interms of molecular weight, molecular weight distribution, melting point,meltflow index, mean isotactic block length, and isotactic index asdefined in terms of mm triads. The polymers produced had isotacticindices, as thus defined, of about 97-98% as contrasted with anisotactic index of 93% for a commercial polypropylene compared with aconventional Ziegler-Natta catalyst and molecular weight distributionsranging from about 2.0 to 3.0 as contrasted with a molecular weightdistribution of 4.5 for the polypropylene produced by the conventionalZiegler-Natta catalyst. Similarly, as in the case of the aforementionedpatent to Peiffer et al, the Canadian '104 application disclosesmultilayer films in which the base ply and one or two top plies can beformed of the same or different propylene polymers including propylenehomopolymers or copolymers or terpolymers. Where a propylene homopolymeris employed in the top ply, it is described as having a melting point ofat least 140° C. Similarly, as in the case of the aforementioned patentto Peiffer et al, the Canadian '104 application discloses multilayerfilms in which the base ply and one or two top plies can be formed ofthe same or different propylene polymers including propylenehomopolymers or copolymers or terpolymers. Where a propylene homopolymeris employed in the top ply, it is described as having a melting point ofat least 140° C. and a melt flow index of 1 to 20 grams/10 minutes. Inthe Canadian '104 application a typical film structure, the base ply ischaracterized as providing at least 40% and typically 50-98% of thetotal film thickness with the outer ply or plies supplying the remainderof the film thickness. Specific overall film thicknesses disclosed inthe Canadian '104 application range from 4 to 100 microns and morespecifically 6 to 30 microns with the base ply specifically ranging from1.5 to 10 microns and the outer plies from 0.4 to 1.5 microns.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a multilayerpolyolefin film of the type suitable for packaging application in whichheat seals are formed. The multilayer film comprises a flexiblesubstrate layer formed of a crystalline thermoplastic polymer having aninterface surface. A heat-sealable surface layer is bonded to theinterface surface of the substrate layer. The surface layer is formed ofa syndiotactic propylene polymer which is effective to produce a heatseal with itself at a sealing temperature of less than 110° C. Thesurface layer has a thickness which is less than the thickness of thesubstrate layer. Preferably, the substrate layer has an averagethickness within the range of 5-150 microns, and the surface layer has athickness which is no more than one-half the thickness of the substratelayer, preferably no more than ⅓ of the thickness of the substrate layerand having a thickness within the range of 0.3-50 microns. Preferably,the heat-seal layer is formed of syndiotactic polypropylene polymerizedin the presence of a syndiospecific metallocene catalyst and having amelt flow index of less than 2 grams/10 minutes. Preferably, themultilayer film is a biaxially-oriented film.

In a further aspect of the invention, there is provided a process forthe production of a multilayer film incorporating a substrate layer anda heat-sealable surface layer. In carrying out the invention, acrystalline thermoplastic polymer is extruded and formed into asubstrate layer film. A second polymer is employed comprising asyndiotactic propylene polymer which is effective to form aheat-sealable surface layer. The propylene polymer is extruded to form asurface layer that is bonded to the interface of the substrate layer ata temperature within the range of 150-260° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration in isometric view of a tenter framesystem that may be employed in forming biaxially-oriented multilayerfilms in accordance with the present invention.

FIG. 2 is a schematic illustration of a tenter frame processincorporating systems for co-extrusion or extrusion coating of surfacelayers bonded to a substrate layer to produce multilayer films inaccordance with the present invention.

FIG. 3 is a graphical presentation of maximum seal strength versus sealtemperature for heat-seal films formed of various polymers.

FIG. 4 is a graphical illustration of average seal strength versus sealtemperature for the polymers illustrated in FIG. 3.

FIG. 5 is a graphical illustration of near term hot seal strength as afunction of seal temperature for the various polymers depicted in FIG.3.

FIG. 6 is a graphical illustration of hot seal strength as a function ofseal temperature for the various polymers depicted in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Heat-sealable multilayer polyolefin films, such as used in packaging offood items and the like, are generally formed by biaxial orientationprocedures, and the invention will be described with respect tobiaxially-oriented films. However, it is to be recognized that theinvention will find application in other multilayer polyolefin films inwhich enhanced heat-seal and hot tack characteristics are desirable.Biaxially-oriented films can be characterized in terms of certainwell-defined characteristics relating to their stereoregular structuresand physical properties, including melt temperatures and shrinkagecharacteristics, as well as in relatively low coefficients of frictionand relatively high tensile moduli and relatively low permeation ratesto oxygen and water. Biaxially-oriented films of the type incorporatingthe present invention are formed with a heat-sealable surface layerincorporating a particular syndiotactic propylene polymer as describedin greater detail below and by using any suitable oriented filmproduction technique, such as the conventionally-used tenter frameprocess.

In general, such biaxially-oriented film production can be carried outby any suitable technique, such as disclosed in the aforementionedCanadian Patent Application No. 2,178,104 to Peiffer et al. As describedin the Peiffer et al application, the polymer or polymers used to makethe film are melted and then passed through an extruder to a slot diemechanism after which it is passed over a first roller, characterized asa chill roller, which tends to solidify the film. The film is thenoriented by stressing it in a longitudinal direction, characterized asthe machine direction, and in a transverse direction to arrive at a filmwhich can be characterized in terms of orientation ratios, sometimesalso referred to as stretch ratios, in both longitudinal and transversedirections. The machine direction orientation is accomplished throughthe use of two sequentially disposed rollers, the second or fast rolleroperating at a speed in relation to the slower roller corresponding tothe desired orientation ratio. This may alternatively be accomplishedthrough a series of rollers with increasing speeds, sometimes withadditional intermediate rollers for temperature control and otherfunctions. After the film has been stressed in the machine direction, itis again cooled and then pre-heated and passed into a lateral stressingsection, for example, a tenter frame mechanism, where it is againstressed, this time in the transverse direction. Orientation in thetransverse direction is often followed by an annealing section.Subsequently, the film is then cooled and may be subjected to furthertreatment, such as corona treatment or flame treatment, as described,for example, in the aforementioned Canadian Patent Application No.2,178,104 or in U.S. Pat. No. 4,029,876 to Beatty, the entiredisclosures of which are incorporated herein by reference. The film mayalso be metallicized as described in U.S. Pat. No. 4,692,380 to Reid,the entire disclosure of which is incorporated herein by reference.While corona and flame treatment typically occurs immediately followingorientation and prior to the initial roll up, metallicizing is typicallyperformed at a separate time and location.

Multilayer oriented films comprise a substrate layer, sometimes termed a“base layer” or “core layer,” formed of a stereoregular propylenepolymer, typically isotactic polypropylene homopolymer, chosen for goodstiffness and other physical properties with one or more thinner surfacelayers used for heat-sealing as well as to provide other properties suchas improved slip or barrier qualities, etc. Numerous methods exist forproducing multilayer films including coextrusion, extrusion coating,extrusion laminating, or standard lamination techniques.

Heat-sealing can be accomplished by placing the heat-sealing surfacelayer in contact with a corresponding layer normally having the same orsimilar chemical make-up as the heat-sealing layer and using acombination of heat and pressure to create a seal bonding the twocorresponding layers together. The heat-seal layer will be a surfacelayer in order to be able to contact and seal with another layer (orwith a different section of itself). After sealing, it is possible thatthe sealed structure may constitute an internal layer in an even morecomplex multilayer film or composition of multilayer films.

The efficacy of a heat-seal layer can be characterized in terms of theheat-seal strength of the product, the seal initiation temperature (SIT)and the so-called “hot tack” characteristic, that is, the hot sealstrength as measured shortly after formation of the laminated filmlayer. Typically, the hot seal strength will be measured at 250milliseconds after formation of the surface layer substrate bond and ata following time interval of 500 milliseconds. The seal initiationtemperature is the temperature at which the bonding of the surface layerto the corresponding layer begins to occur. Failure of a heat seal canoccur through a number of mechanisms that can be characterized in termsof “peel,” “web stretch,” or “tear failure.” Failure of a heat seal dueto peel is characterized by a seal peeling apart at the interface.Failure due to web stretch occurs due to a differential strength betweenthe web and the heat-seal. Failure because of tear involves a tearing ofthe web itself at the seal edge.

The heat-sealable surface layer is typically formed by coextrusion ofthe surface layer polymer with the substrate layer polymer. Co-extrusioncan be carried out by simultaneously coinjecting the polymer of theheat-seal layer and the polymer of the substrate layer through a slotteddie system to form a film formed of an outer layer of the heat-sealingpolymer and substrate layer of the core polymer. Additional layers canalso be coextruded, either as an additional heat-seal layer on the othersurface of the substrate layer, or layers serving other functions, suchas barriers, anti-block layers, etc. Alternatively, a heat-seal layercan be extrusion coated later in the film making process. Also, otherlayers can be added to create a more complex film after orcontemporaneous with the formation of the basic heat-seal layer to corelayer relationship. The advantages of the present invention remain solong as the heat-seal layer is contiguous to and bonded with thesubstrate layer.

Turning now to FIG. 1, there is shown a schematic illustration of asuitable “Tenter Frame” orientation process that may be employed inproducing biaxially-oriented polypropylene film in accordance with thepresent invention. More particularly and with reference to FIG. 1, asource of molten polymer is supplied from a hopper 10 to an extruder 12and from there to a slot die 14 which produces a flat, relatively thickfilm 16 at its output. Film 16 is applied over a chill roller 18, and itis cooled to a suitable temperature within the range of about 30°-60° C.The film is drawn off the chill roller 13 to a stretching section 20 towhich the machine direction orientation occurs by means of idler rollers22 and 23 that lead to preheat rollers 25 and 26.

As the film is drawn off the chill roller 18 and passed over the idlerrollers, it is cooled to a temperature of about 30°-60° C. In stretchingthe film in the machine direction, it is heated by preheat rollers 25and 26 to an incremental temperature increase of about 60°-100° C. andthen passed to the slow roller 30 of the longitudinal orientingmechanism. The slow roller may be operated at any suitable speed,usually about 20-40 feet per minute. The fast roller 31 is operated at asuitable speed, typically about 150 feet per minute, to provide asurface speed at the circumference of about 4-7 times that of the slowroller in order to orient the film in the machine direction. As theoriented film is withdrawn from the fast roller, it is passed overroller 33 at room temperature conditions. From here it is passed overtandem idler rollers 35 and 36 to a lateral stretching section 40 wherethe film is oriented by stretching in the transverse direction. Thesection 40 includes a preheat section 42 comprising a plurality oftandem heating rollers (not shown) where it is again reheated to atemperature within the range of 130°-180° C. From the preheat section 42of the tenter frame, the film is passed to a stretching or draw section44 where it is progressively stretched by means of tenter clips (notshown) which grasp the opposed sides of the film and progressivelystretch it laterally until it reaches it maximum lateral dimension.Lateral stretching ratios are typically greater than machine directionstretch ratios and often may range from 5-12 times the original width.Lateral stretching ratios of 8-10 times are usually preferred. Theconcluding portion of the lateral stretching phase includes an annealingsection 46, such as an oven housing, where the film is heated at atemperature within the range of 130°-170° C. for a suitable period oftime, about 1-10 seconds. The annealing time helps control certainproperties, and increased annealing can be used specifically to reduceshrinkage. The biaxially-oriented film is then withdrawn from the tenterframe and passed over a chill roller 48 where it is reduced to atemperature of less than about 50° C. and then applied to take-up spoolson a take-up mechanism 50. From the foregoing description, it will berecognized that the initial orientation in the machine direction iscarried out at a somewhat lower temperature than the orientation in thelateral dimension. For example, the film exiting the preheat rollers isstretched in the machine direction at a temperature of about 120° C. Thefilm may be cooled to a temperature of about 50° C. and thereafterheated to a temperature of about 160° C. before it is subject to theprogressive lateral dimension orientation in the tenter section.

From the foregoing description it will be recognized thatbiaxially-oriented film can have a number of properties to its advantageduring and after the machine processing steps. A relatively lowcoefficient friction is desirable, both during the biaxially orientationprocedure and in the end use applications of the ultimately-producedbiaxially-oriented film. A relatively high stiffness, as indicated bythe tensile modulus in both the machine direction and the transversedirection is usually advantageous. Relatively low permeabilities to gasand water are desirable in many applications. In addition, a highshrinkage factor of the processed film, while undesirable in some cases,can be advantageous in other applications, such as where the film isused in stretch wrapping of food products, electrical components, andthe like.

FIG. 2 is a schematic diagram illustrating a tenter-frame processcarried out with the co-extrusion of one or two surface layers with asubstrate layer. The main extruder 100 is flanked by two supplementalextruders 102 and 104. Through the operation of one of the supplementalextruders 102 or 104, a separate polymer or polymer blend may beextruded to be in contact with the primary polymer or polymer blendforming the substrate supplied from the main extruder 100. If bothsupplemental extruders 102 and 104 are used, then a sandwich may becreated with the primary polymer forming the core or substrate layer,and the polymers extruded by the supplemental extruders 102 and 104forming surface layers. After extrusion and casting, the multilayer filmcontinues through the machine direction orientation section 106,pre-heating section 108, transverse direction orientation section 110,annealing section 112, cooling section 114, corona treating section 116,and finally the take-up (or wind-up) section 118. In an alternative modeof operation, on one or more surface layers may be added in extrusioncoating section 120, after machine direction orientation, but beforetransverse direction orientation. In extrusion coating section 120,additional material is extruded to coat either one or both surfaces ofthe mono-axially oriented film emerging from machine directionorientation section 106. The mono-axially oriented film supplied to beextrusion coated may be a mono-layer film generated by primary extruder100, or may be a multilayer film created by co-extrusion by acombination of main extruder 100 and supplemental extruders 102 and/or104.

The syndiotactic propylene polymer employed in forming the heal seallayer of the present invention can be characterized by a low sealinitiation temperature (SIT) of less than 100° C. and effective sealstrength characteristics at relatively low seal temperatures of about110° C. or less. The seal initiation temperatures and the lowtemperature heat-seal strengths are substantially less than thecorresponding values observed for isotactic polypropylene conventionallyused in forming heat-seal layers. In fact, the SIT and heat-sealstrength characteristics, together with the hot tack properties of thesyndiotactic polypropylene, are generally better for the syndiotacticpolypropylene film than for corresponding films produced with ethylenepropylene copolymers.

The syndiotactic polypropylenes employed in the present invention areproduced by the polymerization of propylene in the presence of asyndiospecific metallocene catalyst of the types disclosed in U.S. Pat.No. 4,892,851 to Ewen et al, U.S. Pat. No. 5,225,500 to Elder et al, andU.S. Pat. No. 5,243,002 to Razavi. The syndiospecific metallocenes maybe employed as homogeneous catalyst systems, or they may be employed assupported catalyst systems as disclosed, for example, in U.S. Pat. No.5,807,800 to Shamshoun et al. For a further description of suitablesyndiospecific metallocene catalyst systems which can be employed in thepolymerization of propylene to produce syndiotactic polypropylene,reference is made to the aforementioned patents to Ewen et al, Elder etal, Razavi, and Shamshoum et al, the entire disclosures of which areincorporated herein by reference.

As described in greater detail below, the syndiotactic polypropyleneemployed in forming the heat-seal layer of the present invention ischaracterized by a melt flow index which is less, usually substantiallyless, than the various other crystalline polymers or copolymers usefulin forming heal seal layers. As a practical matter, the syndiotacticpolypropylene is characterized by a melt flow index of less than 3grams/10 minutes and preferably less than 2 grams/10 minutes. The meltflow index is characterized as the melt flow rate as determined inaccordance with ASTM Standard D-1238 at 230° C. using 2.16 kilograms offorce.

The heat-seal layer (or layers) and substrate layer are normallyprovided in configurations in which the surface layer has a thicknesssubstantially less than the thickness of the substrate layer. Fortypical packaging applications, the substrate layer will exhibit anaverage thickness within the range of 5-150 microns. The heat-seal layerwill have a thickness of no more than one-half the thickness of thesubstrate layer and usually less than ⅓ of the substrate layer. Thesurface layer typically will have a thickness within the range of 0.3-50microns.

The substrate layer may be formed of various polymers or polymer blendsas described previously. Isotactic polypropylene homopolymers orpropylene/ethylene copolymers, typically containing no more than 10 wt.% ethylene, may be used to form the substrate layer. A preferredsubstrate layer incorporating polypropylene having a very highisotacticity is defined in terms of meso pentads and meso diads but alsohaving irregularities in the polymer structure characterized in terms of2,1 insertions as contrasted with the predominate 1,2 insertionscharacteristic of isotactic polypropylene. Thus the polymer chain of theisotactic polypropylene is characterized by intermittent head to headinsertions to result in a polymer structure as exemplified below.

As shown by the polymer structure depicted by Formula (5), theoccasional head-to-head insertion resulting from the use of the 2-alkylsubstituted indenyl group results in adjacent pendant methyl groupsseparated by ethylene groups resulting in a polymer structure whichbehaves, somewhat in the fashion of a random ethylene propylenecopolymer and results in a variable melting point. This results in apolymer which can be advantageously employed to produce abiaxially-oriented film having good characteristics in terms of strengthin both the machine and transverse directions, low co-efficients forfriction, and relatively low permeabilities to water and to oxygen. Atthe same time, the biaxially-oriented films thus produced havesatisfactory haze properties, normally less than 1%, and good glosscharacteristics, greater than 90%. This polymer can be prepared by thepolymerization of propylene in the presence of a metallocene catalystcharacterized by the formularac-R′R″Si(2-RiInd)MeQ₂  (6)In Formula (6), R′, R″ are each independently a C₁-C₄ alkyl group or anphenyl group; Ind is an indenyl group substituted at the proximalposition by the substituent R₅ and otherwise unsubstituted; Ri is anethyl, methyl, isopropyl, or tertiary butyl group; Me is a transitionmetal selected from the group consisting of titanium, zirconium,hafnium, and vanadium; and each Q is independently a hydrocarbyl groupor containing 1 to 4 carbon atoms or a halogen.

As indicated by Formula (6) above, the silyl bridge can be substitutedwith various substituents in which R′ and R″ are each independently amethyl group, an ethyl group, a propyl group (including an isopropylgroup), and a butyl group (including a tertiary butyl or an isobutylgroup). Alternatively, one or both of R′, R″ can take the place of aphenyl group. Suitable bridge structures are dimethylsilyl,diethylsilyl, and diphenylsilyl structures.

The Ri substituent at the 2 position (the proximal position with regardto the bridge head carbon atom) can be a methyl, ethyl, isopropyl, ortertiary butyl. Preferably, the substituent at the 2 position is amethyl group. As noted previously the indenyl group is otherwiseunsubstituted except that it may be a hydrogenated indenyl group.Specifically, the indenyl ligand can take the form of a 2-methyl indenylor a 2-methyl tetrahydroindenyl ligand. As will be recognized by thoseskilled in the art, the ligand structure should be a racemic structurein order to provide the desired enantiomorphic site control mechanism toproduce the isotactic polymer configuration.

As described previously, the 2,1 insertions produce “mistakes” in thepolymer structure which impart the desired non-uniform melting pointcharacteristics of the present invention. The corresponding film ischaracterized in terms of low water and oxygen permeabilities and lowcoefficients of friction as described hereinafter. The “mistakes” due tothe 2,1 insertions should not however be confused with mistakesresulting in racemic insertions as indicated, for example, by thefollowing polymer structure:

As will be recognized, the structure (7) can be indicated by the pentadmrrm. The “mistakes” corresponding to the head-to-head insertionmechanism involved in the present invention are not characterized by orare not necessarily characterized by racemic diads.

In experimental work carried out with respect to the present invention,the seal strength and hot tack characteristics of films formed ofsyndiotactic polypropylene were evaluated against films formed ofisotactic propylene homopolymers and propylene/ethylene copolymers. Thesyndiotactic polypropylene was prepared by the polymerization ofpropylene in the presence of a syndiospecific bridged metallocene of thetype disclosed in U.S. Pat. No. 4,892,851 to Ewen. Exemplary of suchmetallocene catalyst systems are metallocenes based uponcyclopentadienyl fluorenyl ligand structures such as isopropylidene(cyclopentadienyl fluorenyl) zirconium dichloride employed with acocatalyst such as alumoxane. Such syndiotactic polypropylenes can alsobe prepared through the use of so-called “cationic” metallocenes thatincorporate a stable non-coordinating anion and do not normally employthe present of an alumoxane. Syndiospecific cationic metallocenes aredisclosed, for example, in the aforementioned U.S. Pat. No. 5,243,002.The syndiotactic polypropylene employed in the experimental workdiscussed below had a melt flow index of 1.5 g/10 min. and had a microstructure characterized by about 80% syndiotactic pentads (rrrr).

The isotactic polymers employed in the experimental work included bothpropylene homopolymers and propylene ethylene copolymers prepared bycatalysis with isospecific metallocenes as disclosed in theaforementioned patents to Ewen and Ziegler-Natta catalysts as disclosedin the aforementioned patents to Mayr et al. The characteristics of themetallocene-based propylene homopolymers (identified herein as MP-1through MP-4), the metallocene-based propylene ethylene copolymer(identified herein as MEP-5), the Ziegler-Natta homopolymer (identifiedherein as ZP-7), and the Ziegler-Natta propylene/ethylene copolymers(identified herein as ZEP-8 and ZEP-6) are set forth in Table I. InTable I the polymers are characterized in terms of melt flow index, NFI(the melt flow rate in grams/10 minutes), the ethylene content (EC) inwt. % where applicable, the xylene solubles content (XS) of thepolymers, and, where available, the isotactic index I as indicated bythe percent of meso pentads. TABLE I Polymer MFI EC I XS MP1 5.5 94.0 .3MP2 13.7 94.1 .4 MP3 9.4 94.0 .5 MP4 3.3 96.3 .2 MEP5 5.0 0.4 95.8 .7ZEP6 4.9 6.8 9.9 ZP7 8.9 3.2 ZEP8 5.0 6.1 10.0

In the experimental work seal strength and hot tack characteristics ofthe syndiotactic polypropylene and in the above-identified polymers wereevaluated on cast films of 50 microns. In carrying out the experimentalwork, the seal strengths were evaluated on films formed at temperaturesstarting below the seal initiation temperature to temperaturesindicating a plateau in the seal strength characteristics.

In FIG. 3, the maximum seal strength (SM) in newtons per centimeter isplotted on the ordinate versus the seal temperature T in degreesCentigrade on the abscissa for some of the above-described polymersystems. In FIG. 3, the curves are designated by the reference numeralsSP for the syndiotactic polypropylene and by the designations shown inTable 1 for the several isotactic propylene homopolymers or copolymers.

FIG. 4 shows graphs of average seal strength SA plotted on the ordinateversus the seal temperature in C° plotted on the abscissa. From anexamination of FIGS. 3 and 4, it can be seen that the syndiotacticpolypropylene film and the films formed with the Ziegler-Natta-basedcopolymers ZEP-6 and ZEP-8 exhibited much lower seal initiationtemperatures (SIT) than the Ziegler-Natta homopolymers. Syndiotacticpolypropylene exhibited a SIT of about 94° C., 6-7° below that of thecopolymer ZEP-6, the highest ethylene content copolymer tested. TheZiegler-Natta copolymer had a somewhat higher seal initiationtemperature of about 108° C., and as indicated, the seal temperatures ofthe remaining polymers were much higher, indicating SIT values of about120-130° C. or even higher. As further indicated by the heal sealstrength, the syndiotactic polypropylene develops maximum seal strengthover a relatively broad plateau in the low temperature range. Whiletemperatures within the range of 95-115° C. can be employed to achieveeffective heal seals, maximum results are achieved within the range of100-110° C.

The hot tack performance of the syndiotactic polypropylene and the otherpolymers tested is shown in shown in FIGS. 5 and 6, which are graphs ofthe hot seal strength Sh in newtons per centimeter plotted on theordinate versus the seal temperature T in ° C plotted on the abscissa.In each of FIGS. 5 and 6, the curves showing the hot tackcharacteristics of the polymers are designated by the same referencecharacters as found in Table 1 and used in FIGS. 3 and 4. FIG. 5indicates the hot seal strength, as measured at 250 milliseconds afterbonding and FIG. 6, the hot seal strength at 500 milliseconds afterbonding. As can be seen from an examination of FIGS. 5 and 6, thesyndiotactic polypropylene film demonstrated hot tack performance whichis substantially superior to the (metallocene-based and)Ziegler-Natta-based propylene/ethylene copolymers. In the hot tackperformance of the remaining polymers, the MiPP homopolymers, as well asthe Ziegler-Natta homopolymers, they are indicated substantiallyinferior in hot tack performance. In fact, at a seal temperature of 105°C., where the Ziegler-Natta-based propylene/ethylene copolymer begins toshow a maximum seal strength greater than the syndiotactic propylene,the syndiotactic polypropylene film still shows far superior hot tackperformance as measured in the near term and also the long term.

Having described specific embodiments of the present invention, it willbe understood that modifications thereof may be suggested to thoseskilled in the art, and it is intended to cover all such modificationsas fall within the scope of the appended claims.

1-20. (canceled)
 21. A process for the production of a multilayer filmhaving a substrate layer and a surface layer comprising: (a) providing afirst crystalline thermoplastic polymer; (b) extruding the propylenepolymer and forming the polymer into a flexible substrate layercomprising an interface surface; (c) providing a second polymercomprising a syndiotactic propylene polymer comprising a melt flow indexof less than 2 grams/10 minutes produced by the polymerization ofpropylene in the presence of a syndiospecific metallocene catalysteffective to form a surface layer, the surface layer capable ofproducing a heat seal with itself at a seal temperature less than 110°C.; (d) extruding the syndiotactic propylene polymer to form a surfacelayer; and (e) bonding the surface layer to the interface surface of thesubstrate layer to form a multilayer film having a surface layer ofsyndiotactic propylene polymer which has a thickness that is less thanthe thickness of the substrate layer.
 22. The process of claim 21,wherein the first polymer is an isotactic propylene polymer.
 23. Themethod of claim 21, wherein the substrate layer film is formed byorienting the substrate layer form in at least one direction andthereafter forming the surface layer by extrusion-coating thesyndiotactic polypropylene on to the oriented substrate layer film. 24.The process of said claim 1, wherein said multilayer film is formed byco-extruding the first and second polymers through a slotted die systemto form a multilayer film comprising a substrate layer of the firstpolymer and a surface layer of the second polymer and thereafterorienting the film in the machine direction followed by orienting thefilm in the transverse direction to form a biaxially-oriented multilayerfilm.
 25. A process for the production of a multilayer film having asubstrate layer and a surface layer comprising: (a) providing a firstpolymer to form the substrate layer of a multilayer film; (b) providinga second polymer comprising a syndiotactic propylene polymer comprisinga melt flow index of less than 2 grams/10 minutes produced by thepolymerization of propylene in the presence of a syndiospecificmetallocene catalyst effective to form a heat-sealable surface layer ofsaid multilayer film; and (c) co-extruding said first and secondpolymers through a slotted die system at a temperature within the rangeof 150′-260° C. to form a film comprising a substrate layer of saidfirst polymer and a surface layer of said second polymer of a thicknesswhich is less than the thickness of said substrate layer.
 26. Theprocess of claim 25, wherein the surface layer of said second polymer iseffective in producing a heat seal with itself at a seal temperature ofno more than 110° C.